While the subject invention may be described quite simply, it can best be understood if the principles, discoveries and analyses that led up to it are understood first. This requires substantial explanation of typical roller clutches and the types of springs that they currently use. Relative to this prior art:
FIG. 1 shows a typical clutch cage with part of the side rail broken away to show a roller and the space where a spring would go;
FIG. 2 shows an accordion spring of a type in which the end leaves are not squared off;
FIG. 3 shows such a spring tipped to the left prior to installation;
FIG. 4 shows the first embodiment installed with the roller in a shipping position;
FIG. 5 shows the roller moved back to a nominal operating position of the roller, and the spring more compressed;
FIG. 6 shows the roller moved back to the limit, and the spring fully compressed.
Referring first to FIG. 1, roller clutches, a typical example of which is indicted generally at 10, have a generally cylindrical cage 12. Cage 12 is installed in the annular space between a pair of clutch races, not illustrated. Cage 12 usually includes a series of box shaped pockets 14 disposed circumferentially about the cage axis. Pockets 14 may be separate units linked together, or may, as in cage 12, be formed between axially spaced side rails 16 and 18 joined together by cross bars 20 and 22. Either way, the pocket is recognizable as a basic rectangle, with a generally flat base, provided here by the cross bar 20 parallel to the axis of the cage 12, and with spaced, parallel sides perpendicular to the base, provided here by the side rails 16 and 18. Each pocket 14 contains a cylindrical roller 24 which travels back and forth in the pocket, generally parallel to the base cross bar 20. As it travels, each roller 24 would be urged continually away from its pocket base 20 by an energizing spring, which is not shown in FIG. 1, but which would fit into the space indicated schematically at 26. The spring keeps roller 24 in a "ready" position, ready to jam between the races.
The energizing spring used is almost always a compression spring, and often is an accordion spring. By accordion spring, it is meant that the spring consists of a series of V shaped loops, each loop comprising a pair of flat leaves joined together at a fold or pleat. There will be two end leaves, so the number of complete loops will equal the number of leaves less one, divided by two. Since the end leaves are not joined to an adjacent leaf to make another loop, they each have a free edge. Accordion springs are installed in the pocket with their front end leaf compressed against a roller, and with their back end leaf compressed against the pocket base. Often, there is no latching structure on the spring, and it retains itself to the cage simply by the fact of its compression, although the pocket base is often U shaped in cross section, like cross bar 20 is above, so as to radially trap the back end leaf of the spring. There are two types of accordion springs, those in which the pleats are axially disposed, and so face the clutch races, and those in which the pleats are radially disposed, and so face the inner surfaces of the pocket side rails instead. As the rollers travel back and forth, the springs compress and expand, which can occur very rapidly. The pleats are crucial to the operation of the spring, being the source of the spring's resilience, and it has been recognized in the art that it is important to protect the pleats from wear against those parts of the clutch that they face.
Different approaches may be found to protecting the pleats of these two different types of accordion springs. In the case of accordion springs with axially disposed pleats, it has been proposed to provide L shaped tabs bent back from the end leaves of the springs to ride on the surfaces of the races and prevent the pleats from rubbing on the races. Examples may be seen in German Auslegeschrift Nos. 1213177 and 1254916. Likewise, in U.S. Pat. No. 3,087,588 to Fischer, assigned to the assignee of the present invention, a shelf like portion of the metal side rails hangs over the upper pleats of the spring, preventing them from rubbing on the outer race. However, spring metal would still be rubbing on cage metal, if the spring were to be thrown centrifugally outwardly. A different approach that entirely avoids the rubbing of the spring pleat on anything may be seen in U.S. Pat. No. 4,711,330 to Lederman, assigned to the assignee of the present invention, which shows a spring with axially disposed pleats in one embodiment, see FIG. 6. Tabs on the front end leaf of the spring ride on side rails of the cage, which rigorously guides the spring and keeps the pleats away from the clutch races. While the spring tabs deliberately rub on the cage side rails, the side rails are plastic, and the tabs are not a crucial, active part of the spring, as the pleats are.
In the case of accordion springs with radially disposed pleats, the problem is to prevent the pleats from rubbing on the inner surfaces of the pocket side rails. Recognition of this problem may be seen in U.S. Pat. No. 4,368,809 to Husmann, where undefined "transverse forces" are said to cause the pleats (called folds there) to butt against the side rails, where they rub and wear. Husmann's solution is to make the pleat to pleat width of the spring significantly smaller than the pocket width, and then to keep the spring centered between the side rails so that it is not physically possible for the pleats on either side of the spring to reach the side rail. Husmann centers the spring by trapping a tab bent off of the front leaf of the spring between the end of the roller and the side rail. The problem of keeping the spring from rubbing on the side rails is also recognized in U.S. Pat. No. 4,664,237 to Lederman et al, also assigned to the assignee of the present invention. There, accordion springs both with axially and radially directed pleats are shown, although the springs have two branches each. Lederman et al discloses centering either type of accordion spring by using the back end, rather than the front end of the spring. At best seen in FIG. 6, the branches of the spring are placed inboard on a spring base far enough that, when the spring base is anchored between the side rails, it is not physically possible for the outside pleats or edges of the spring to rub on the side rails. So, in both Husmann and Lederman et al, an external anchoring force is used to keep the spring away from the side rails.
An area that apparently has not received much recognition or analysis in the published prior art is the exact nature of those "transverse" forces that Husmann generally refers to above, the forces which in fact cause the pleats of an accordion spring to butt against the inner surfaces of the side rails and rub. More about the nature of these transverse forces was discovered by the current inventor in the course of modifying the way accordion springs with radial pleats are manufactured and installed. How such a spring was conventionally manufactured and installed may be seen clearly in FIG. 3 of Husmann. Note that both end leaves of the spring have been bent in to square off the spring. That same squaring off of the end leaves of a spring with radially directed pleats may be seen in U.S. Pat. No. 4,724,940 to Lederman, assigned to the assignee of the present invention, see FIG. 5. Also, in U.S. Pat. No. 4,664,237 to Lederman, noted above, the radial pleat embodiment of the double branched spring has its back end leaves, although not its front end leaves, squared off. The obvious purpose for squaring off at least the back end leaf of an accordion spring with radial pleats is to ease installation of the spring. That is, the spring will then fit squarely between the roller and the pocket base. If the end leaves were not squared of, then one would be trying to fit a spring with a rhomboid perimeter within a basically rectangular pocket, rather than fitting a rectangle within a rectangle. An incidental result of squaring off the end leaves of such a spring is that the center, active loops of the spring are thereby oriented basically symmetrically to the axis of the cage. This may be seen clearly in U.S. Pat. No. 4,664,237, FIG. 7, where the dot-dash line indicating the central plane of the spring would be square to the cage axis. However, another consequence of squaring off the end leaves of an accordion spring is that the spring is no longer totally symmetrical, as the subtended angle at the pleat of the two end loops is decreased. Since it is the pleats that create the resilience in the spring, a different angle for some or all of them leads to unbalanced stresses in the spring as it is compressed by the moving roller.
This realization led to the use of a modified spring, shown in FIG. 2 and indicated generally at 28. Spring 28 is an accordion spring of the type described above, and fits within the spring space 26 of cage 12. Spring 28 also has V shaped loops formed by flat leaves joined at radially disposed pleats. Specifically, spring 28 has inner leaves 30 of identical length, joined at left side pleats 32 and at right side pleats 34 to create V shaped loops. A significant difference, however, is that the back end leaf 36 and front end leaf 38 are not squared off. Therefore, every V shaped loop subtends an identical angle, and they are all symmetrically disposed about the central plane shown by the dot-dash line. Consequently, when the spring 28 is compressed, the loops formed by the leaves 30 will be stressed equally to the loops formed by the end leaves 36 and 38, unlike a conventional spring with squared off end loops. Spring 28 is also easier to manufacture, in that the squaring off is eliminated. As best seen in FIG. 3, to install spring 28 between the pocket base 20 and roller 24, it has to be first tipped or tilted to the left so that the front end leaf 38 can be placed squarely against roller 24 and so that back end leaf 36 can be placed squarely against pocket base 20. A squared off, conventional spring does not have to be tipped, of course. In tilting the spring 28 to the left, the left side pleats 32 are shifted to the left. The forwardmost part of spring 28, the free edge of the front end leaf 38, shifts the most to the left. To avoid any chance of the free edge of front end leaf 38 hitting the left hand side rail 16, both end leaves 36 and 38 are deliberately made significantly shorter than the other leaves 30. Shortening both end leaves 36 and 38 was felt to be very important, as the spring 28 could then be installed in either direction, and scraping of the front end leaf 38 free edge still avoided. So, spring 28 was easier to make, had equal stresses in all loops, and was not significantly more difficult to install as a practical matter than was a squared off spring.
Despite the advantages of spring 28, another consequence was discovered. As may be seen in FIGS. 3 through 6, when spring 28 is tipped, its loops are disposed non symmetrical to the axis of cage 12, and the central plane of spring 28, along which its compressive force acts, is not perfectly perpendicular to roller 24 or pocket base 20. Therefore, as roller 24 compresses spring 28, a component of the compressive force will act in the opposite axial direction to the direction in which spring 28 was tipped when it was installed, that is, to the right. That sideways component may be termed the side thrust. The side thrust would be reversed if spring 28 were installed and tipped the other way, which is possible, as the front and back of spring 28 are arbitrary. While the percentage of compressive force acting to the right decreases as the spring 28 compresses more, the magnitude of the force is greater.
Referring to FIGS. 4 through 6, as the roller moves toward the base cross bar 20, the side thrust will shift spring 28 to the right, bringing the right side pleats 34 up against the inner surface of the right side rail 18. The lower right side pleats 34 will hit first, FIG. 4, and as spring 28 compresses further, all of the right side pleats 34 will eventually hit, FIG. 6. The free edge of the back end leaf 36 will not hit at all, as it is shorter than the other leaves 30. With the right side leaves 34 kept continually against the inner surface of the right side rail 18 as the roller 24 traveled back and forth, the potential for rubbing wear can be seen. While excessive wear can be avoided by using tough spring material or plastic side rails, or both, it would be desirable to avoid the rubbing altogether, if possible.